To reap the above benefits, two research lines have been identified, namely IRS-aided simultaneous wireless information and power transfer (SWIPT) and IRS-aided wireless powered communication networks (WPCNs). Specifically, the first line of research aims at exploiting the high passive beamforming gain to enlarge the rate-energy tradeoff in IRS-aided SWIPT systems where information and energy receivers are served concurrently using the same RF signals sent from an access point (AP) [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}. To this end, subject to the minimum signal-to-interference-plus-noise ratio (SINR) requirements of information receivers, joint information and energy beamforming design was studied in [1]} to maximize the weighted sum-power of energy receivers. It was later extended in [2]} and [7]} by considering the transmit power minimization and sum-rate maximization problems, respectively. In particular, the results in [2]} showed that the use of IRS not only lowers the transmit power required at the AP but also effectively reduces the number of energy beams as compared to the case without IRS, which thus greatly simplifies the transmitter design. In contrast, IRS-aided WPCNs focus on improving the communication performance by exploiting IRS to assist WPT and WIT across different time slots. It is mainly based on a “harvest and then transmit” protocol where self-sustainable devices first harvest energy in the downlink (DL) and then transmit information in the uplink (UL) [16]}, [17]}, [18]}, [19]}. The sum throughput of an IRS-aided WPCN was maximized in [16]} for UL WIT employing time-division multiple access (TDMA). Then, the common throughput maximization problem of an IRS-aided WPCN with user cooperation in the UL was studied in [17]}. However, this study was limited to a WPCN with two users and also increases the coordination complexity. In [18]}, the extension to the multiuser case was presented, where space-division multiple access (SDMA) was employed for UL WIT by jointly optimizing the IRS phase shifts and transmit powers.
Despite of the above works, some fundamental issues still remain unsolved in IRS-aided WPCNs.
First, does exploiting more IRS phase-shift patterns/vectors over time for DL WPT and UL WIT really bring throughput improvement of a WPCN? Since DL WPT and UL WIT have different design objectives and also occur in different time periods, it is usually believed that exploiting dynamic IRS beamforming, i.e., adopting different IRS phase-shift vectors in the above two phases, is able to improve the system performance. As such, all the above works on IRS-aided WPCNs naturally assumed that different phase-shift vectors are adopted for DL WPT and UL WIT, respectively, and then solved the corresponding problems numerically with suboptimal solutions, which, however, does not provide any concrete insights into this issue. Therefore, it still remains an open problem when dynamic IRS beamforming is actually beneficial for maximizing the throughput of WPCNs. Second, how to jointly optimize the dynamic IRS beamforming and system resource allocation for an arbitrary number of phase-shift vectors? This question is motivated by the fact that even if dynamic IRS beamforming is able to improve the performance, it also incurs more signalling overhead. Specifically, due to the limited computing capability of the low-cost IRS, the hybrid access point (HAP) is typically in charge of the algorithmic computations and then sends the optimized phase shifts to the IRS controller for reconfiguring reflections. As such, adopting more IRS phase-shift vectors not only increases the computational complexity due to more optimization variables, but also leads to more signalling overhead as well as the associated delay for feeding them back to the IRS controller. As such, it may not be preferable to excessively rely on dynamic IRS beamforming considering the performance-cost tradeoff, especially when the number of IRS's elements is practically large.
